Detect corner frame on form

hello, I would like to detect a specific corner in a form which is scanned. The aim is to determine the upper-left corner (left corner of the form's frame). I try to do this with canny edge detector + harris, but I get many corner. So, do everyone has an idea to isolate the corner that I want to get?<<http://hpics.li/60f49cf>>

Answer 1

 edepoints( unique(convhulln(edgepoints))   , :)